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#17. People with Down syndrome may see the world differently

Research suggests that all children with Down syndrome experience impaired vision to varying degrees. Early detection, treatment and effective
support are vital to minimize the impact of poor eyesight on development and learning. Teachers and parents should be aware of adaptations
that can help children with Down syndrome see more clearly.

What research has shown

Children's vision develops rapidly in their early years and it is thought to be influenced by both genetics and the visual input provided
as children look at and move about their world. Vision depends on the functioning and development of the eyes, the optic nerve pathways from
eyes to brain and the visual cortex. A child's vision is going to affect all aspects of their development and their academic progress
in school. Over the last 25 years, research has provided important insights into the visual development of children with Down syndrome.[
1] Much of this research has been conducted by Dr Maggie Woodhouse OBE and her colleagues at Cardiff University in the UK.[2-
4,6,7]

There are a number of aspects of vision which influence how well we see and are evaluated in a full assessment of a child's vision. These
include the ability to see objects over a range of distances (a school child with normal vision can see objects clearly from 7cm to far distance
- this changes with age and typically older eyes are not so good at near distances so reading glasses are needed). If a child has difficulty
seeing close up but can see at distance they are longsighted (hyperopia) and if they see well close up but not at distance then they are
short-sighted (myopia). Another focusing error caused by eye shape is astigmatism. These are called refractive errors and can be largely
corrected with spectacles. Research indicates that some 50%-80% of children with Down syndrome have refractive errors but the rate varies
in different ethnic populations and tends to increase with age during school years.[1] In infancy their vision is not
different from other children. However, many typically developing children who have early refractive errors improve over time without treatment
during the preschool years as natural changes take place in the dimensions of the eye (the process of emmetropisation). This process does
not seem to take place in the same way for children with Down syndrome.[1,2] It is therefore recommended
that refractive errors should be corrected with spectacles earlier than usual and that many children with Down syndrome can benefit from
bifocals which may not only correct their vision but also improve it for some children so spectacles are no longer needed.[3,
4] The wearing of bifocals has been shown to improve school progress especially for literacy in a Canadian study.[5]

The eye's ability to focus clearly over a range of distances is due to a process called accommodation - the ability to change the shape
of the lens in the eye to focus the image on the retina at the back of the eye. Visual acuity refers to sharpness and clarity of the image.
Research suggests that accurate accommodation is a problem for all children with Down syndrome and even those who do not need spectacles
have impaired acuity.[6,7] The images they see will not be quite so sharp and this has been referred
to as seeing in 'soft focus'. Contrast sensitivity is the ability to see objects or patterns when there is only a small difference
in contrast between objects/patterns in view. This has also been shown to be impaired for many children with Down syndrome[7,
8] and a practical implication is that a pencil line on white paper may not be visible for them. The evidence suggests that less effective
accommodation, reduced acuity and contrast sensitivity are not explained by the way the eye works but are due to impaired processing in
the optic nerves and visual cortex.[1,7,8]

In addition to identifying how well each eye works it is also important to know how well they work together, binocular vision, as it is the
information combined from two eyes looking at the same image from slightly different angles that the brain uses to judge distances (depth
perception or stereopsis). Poor depth perception could affect many aspects of a child's life and physical activities including their
confidence in using stairs and escalators. Children with Down syndrome are more likely to experience the eye movement conditions strabismus
(squint) and nystagmus (eye wobble). Both conditions are more common among children with Down syndrome. Up to 42% of children may have strabismus
and again rates increase with age.[1] It can be treated with spectacles. Strabismus can impair depth perception but
does not always do so. We have little information on the depth perception of children with Down syndrome, but one study indicates that 44%
of the children with strabismus demonstrated stereopsis and as the tests are difficult probably the percentage is higher which is encouraging.[
9] One small study of adults with Down syndrome indicates that they all had impaired depth perception but it is not known if this had
been the case for these individuals in childhood or had developed in adult life.[10]

There are other eye conditions that are more common in children with Down syndrome including cataracts (affecting the lens of the eye), keratoconus
(affecting the cornea of the eye) and blepharitis (inflammation of the eyelids) which need to be diagnosed and treated if they occur.[
1] It is especially important not to miss cataracts in the new born baby with Down syndrome as 1%-2% will have cataracts and early treatment
will improve outcomes. These conditions can occur throughout life.

How this is helping

This research has led to guidelines recommending more frequent eye tests for children with Down syndrome from birth as, if untreated, a number
of these possible eye problems will interfere with visual development and lead to poorer vision over time.[11,12]
One study has demonstrated the benefits of implementing the current guidelines showing children being screened and prescribed spectacles
earlier.[13]

Some problems (refractive errors, strabismus) can be treated with spectacles and with better frames available most children will learn to wear
their spectacles from an early age. Others (visual acuity, contract sensitivity) cannot at present be treated so need to be compensated for
especially in the classroom and for close work.

Useful practical information sheets for families and practitioners on adaptations to teaching materials, font sizes and pens from the Cardiff team are available from the UK
Down's Syndrome Association. Summaries of the Cardiff team's current studies and published findings can be found on their website plus a helpful glossary of the technical terms used to describe visual impairments and advice on how to get children to wear their spectacles.

Unanswered questions

Future research is needed to:

investigate how visual development over time is influenced by visual experience, play, exploration and movement

investigate the benefits of wearing bi-focals and the factors which may affect individual differences in outcomes

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